将节点插入二叉搜索树

Question

我已经在二进制搜索树实现中插入了几天而且我已经知道通过使用我的'insert（）来填充我的root了（我在调试时可以看到这一点，使用Eclipse）。 为什么我的其他节点不会被添加到树中？

这是我的BST课程：

package binarySearchTree;

public class BinarySearchTree<T extends Comparable<T>> {

@SuppressWarnings("hiding")
private class BinarySearchTreeNode<T>{
    public BinarySearchTreeNode left, right;
    private T data; //LINE 8

    private BinarySearchTreeNode (T data,BinarySearchTreeNode left, BinarySearchTreeNode right ) {
        this.left = left;
        this.right = right;
        this.data = data;
    }
}
private BinarySearchTreeNode<T> root;

@SuppressWarnings("unused")
private T search(T target, BinarySearchTreeNode<T> ptr) {
    //find target in subtree A ptr
    if (root == null || ptr == null) {
        return root; //target is not in tree
    }
    int compare = target.compareTo(ptr.data); //compareTo(ptr.data);
    if (compare == 0) {
        return ptr.data; //target is found
    }
    if (compare < 0) {
        return search(target, ptr.left);
    }
    if (compare > 0) {
        return search(target, ptr.right);
    }
    return target;
}
public T search(T target) {
    return search(target);
}
public boolean isEmpty() {
    return root == null;
} 
/* To insert a data into a BST, 1st search for the data, 
 * if the data is found = duplicate data error
 * if the data is NOT found = a null pointer
 * Make this null pointer point to a NewNode holding data
 * new values go into the BST as leaves
 * Using public boolean insert (T node) & 
 * private boolean insert (T Node, BSTNode<T> ptr) as a recursive method
 */
@SuppressWarnings("unchecked")
private boolean insert(T value, BinarySearchTreeNode<T> ptr) {
    //T data = null;
    //insert data in a child of ptr, return false if duplicate is found
    //Precondition: ptr must not be null
    int compare = value.compareTo(ptr.data); //LINE 55
    if (compare == 0) {
        return false;
    }
    if (compare < 0) {
        if (ptr.left == null) {
            //found insertion point
            BinarySearchTreeNode<T> node = new BinarySearchTreeNode<>(value, null, null);
            ptr.left.data = node; //insert data in new node
            return true;
        } 
    } else {
        return insert(value, ptr.left); //LINE 67
    }
    if (compare > 0) {
        if (ptr.right == null) {
            BinarySearchTreeNode<T> node = new BinarySearchTreeNode<>(value, null, null);
            ptr.right.data = node;
            return true;
        } else {
            return insert(value, ptr.right);                    
        }
    }
    return false;
}
public boolean insert(T value) {     
    if (isEmpty()) {              
        root = new BinarySearchTreeNode<T>(value, null, null);  
        return true;  
    }
    return insert(value, root); // LINE 85  
} 
}

这是我的Main（），最后我想在控制台中打印我的BST的值，但首先我知道它们需要添加到树中：

package binarySearchTree;

公共类Main {

public static void main(String[] args) {


    BinarySearchTree<String> bstStrings = new BinarySearchTree<String>();

    String s = "Hello";
    String s1 = "World";
    //String s2 = "This Morning";

    bstStrings.insert(s);
    bstStrings.insert(s1); //LINE 15
    //bstStrings.insert(s2);

    while (true){
        if (!bstStrings.isEmpty()){
            System.out.println(bstStrings + " ");
        }
        System.out.println();
        System.out.println("You should have values above this line!");break;
    }   
}
}

Answer 1

你的root应该是BinarySearchTree<T>而不是T
因此，您不会将值存储在根的子树中。
替换这个：
return insert((T) value, node);
同
return insert((T) value, root);

在您的代码中替换如下：

public boolean insert(T value) {     
    if (isEmpty()) {              
        root = new BinarySearchTreeNode((T) value, null, null);  
        return true;  
    }
    return insert((T) value, root); // start recursion  
}    

否则，您没有树，节点没有相互链接

更新：
你得到了NPE，因为你在第一次比较中传入了insert root的左子项，这是null 。
您不应该返回boolean而是返回BinarySearchTreeNode 。
你的方法应该是：

@SuppressWarnings("unchecked")
private BinarySearchTreeNode<T> insert(T value, BinarySearchTreeNode<T> ptr) {  
   if(ptr == null){  
        ptr = new BinarySearchTreeNode<T>(value,null,null);  
        return ptr;  
    }  
    //your code next but return the `ptr`  
}  

然后在插入中你应该做：

public void insert(T value) {     

    root = insert(value, root); 
}

Answer 2

第一次插入后，您创建新节点，但不对它们执行任何操作。